CHEM 14A Lecture Notes - Electron Affinity, Acid Strength, Gibbs Free Energy

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turquoisemoose666

7 Dec 2020

School

UCLA

Department

Chemistry and Biochemistry

Course

CHEM 14A

Professor

Dr.Laurence Lavelle

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Chemist commonly interpret trends in the properties of compounds by considering structures of their molecules. Two reasons why the relative strengths of acids and bases are difficult to predict from molecular structures. Ka and kb are equilibrium constants, so they are related to the gibbs free energy of the proton transfer reaction. Therefore, their values depend on consideration of entropy as well as enthalpy. The solvent plays a significant role in proton transfer reactions. Acid strengths cannot be expected to be related solely to the acid molecule itself. However, although absolute values are difficult to predict, trends among series of compounds with similar structures and in the same solvent can be expected. Acids composed of hydrogen and one other element, such as hcl and h2s. The enthalpy change for the overall process is therefore the sum of these contributions. A high electron affinity of a suggests a high electronegativity and therefore, a strongly polar h-a bond.

Related Questions

Which of the following does not have a more stable resonance structure?

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Question 21 pts

In the following molecule, which carbons are electron deficient, meaning they hold a partial positive charge (Î´+) in the hybrid structure. All carbon atoms are identified by a number.

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Question 31 pts

Rank the following resonance structures from most to least stable.

III (most stable) > I > II (least stable)

III (most stable) > II > I (least stable)

I (most stable) > II > III (least stable)

II (most stable) > III > I (least stable)

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Question 41 pts

Which of the following accurately depicts a Lewis acid-base reaction where a carbon is acting as the Lewis acid and an oxygen is acting as the Lewis base.

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Question 51 pts

Which of the following statements about Lewis acids and bases is true?

A Lewis base always accepts an electron pair from a Lewis acid.

The interaction between a Lewis acid and a Lewis base creates a covalent bond.

Lewis acids are also commonly called nucleophiles.

A Lewis base always accepts a proton (H+) from a Lewis acid.

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Question 61 pts

A molecular orbital energy diagram was created that contains two bonding orbitals and two antibonding orbitals. How many atomic orbitals were mixed to create all these orbitals?

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Question 71 pts

Which of the following statements about the molecular orbital diagram for H2â is false?

Two atomic orbitals combine to produce two molecular orbitals.

There is one filled bonding orbital.

There is one empty antibonding orbital.

There is a non-zero bond order.

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Question 81 pts

If a p atomic orbital and an s atomic orbital were to interact and combine to create new molecular orbitals, how many antibonding molecular orbitals would be created?

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Question 91 pts

Which of the following contains an aldehyde, an ether and an alcohol? Choose all that apply.

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Question 101 pts

Which of the following contains a carboxylic acid and an ether? Choose all that apply.

B-Oxidation of Trans Fats ljnsaturated fats with trans double bonds are commonly referred to as "trans fats." There has been much discussion about the effects of dietary trans fats on health. In their investigations of the effects of trans fatty acid metabolism on health, Yu and col- Ieagues (2004) showed that a model trans fatty acid was processed differently from its cis isomer. They used three related l8-carbon fatty acids to explore the difference in F oxidation between cis and trans isomers of the same-size fatty acid.

(a) Why did Yu and colleagues need to use CoA deriva- tives rather than the free fatty acids in these experiments?

(b) Why were no lower molecular weight CoA derivatives found in the reaction with stearoyl-CoA?

(c) How many rounds of B oxidation would be required to convert the oleoyl-CoA and the elaidoyl-CoA to cd,s-L5- tetradecenoyl- C oA and trans-A5-tetrade cenoyl- C oA, respectively?

There are two forms of the enzyme acyl-CoA dehydroge- nase (see Fig. l7-8a): long-chain acyl-CoA dehydrogenase (LCAD) and very-long-chain acyl-CoA dehydrogenase (VLCAD). Yu and coworkers measured the kinetic parameters of both enzymes. They used the CoA derivatives of three fatty acids: tetradecanoyl-CoA (C1a-CoA), c'i,s-L5-tetradecenoyl- CoA (cAsC,n-CoA), and trans-L5-tetradecenoyl-CoA (tAsOr4- CoA). The results are shown below. (See Chapter 6 for defini- tions of the kinetic parameters.)

(d) For LCAD, the K^ differs dramatically for the cis and trans substrates. Provide a plausible explanation for this observation in terms of the structures of the substrate mole- cules. (Hint: You may want to refer to Fig. I0-2.)

(e) The kinetic parameters of the two enzymes are rele- vant to the differential processing of these fatty acids only tf the LCAD or VLCAD reaction (or both) is the rate-limiting step in the pathway. What evidence is there to support this assumption?

(f) How do these different kinetic parameters explain the different levels of the CoA derivatives found after incubation of rat liver mitochondria with stearoyl-CoA, oleoyl-CoA, and elaidoyl-CoA (shown in the three-panel figure)?

Yu and coworkers measured the substrate specificity of rat liver mitochondrial thioesterase, which hydrolyzes acyl- CoA to CoA and free fatty acid (see Chapter 21). This enzyme was approximately twice as active with C1a-CoA thioesters as with C 1s-CoA thioesters.

(S) Other research has suggested that free fatty acids can paSS through membranes. In their experiments, Yu and colleagues found trans-L5-tetradecenoic acid outside mito- chondria (i.e., in the medium) that had been incubated with elaidoyl-CoA. Describe the pathway that led to this extramito- chondrial trans-L5-tetradecenoic acid. Be sure to indicate where in the cell the various transformations take place, as well as the enzymes that catalyze the transformations.

(h) It is often said in the popular press that "trans fats are not broken down by your cells and instead accumulate in your body." In what sense is this statement correct and in what sense is it an oversimplification?

I want to find the Kn value of Cu(II) and EDTA. I was given the formula kn= [ML4]/[ML3][L]. 12.3 ml of edta was used to tritrate 0.1194 grams of Copper in 50 ml of water. The indicator used was murexide.

At the core of general acid-base theories, there is the concept of âtransferâ of one entity to another. In the case of BrÃ¸nsted-Lowry acid base theory, the transferred entity is the proton, and their transfer processes are widely exploited in chemical methods. In addition to proton transfer, electron pairs can also be donated/shared between donors and acceptors. In this case, an atom/molecule, which can accept a pair of electrons, is referred to as a Lewis acid while the entity donating the pair of electrons is referred to as a Lewis base. The most prevalent Lewis acid-base systems involve metal complexes with vacant nd-orbitals, as well as electron deficient systems including B, Si, Ge, etc. The Lewis acid base theory is also known as the Hard-Soft acid base theory (HSABT) with âhardnessâ defined as small highly charged species that are weakly polarizable (Lewis acids). In contrast, âsoftnessâ is described by species that are larger and highly polarizable species with low charge (Lewis bases).
In the case of metal chemistry, Lewis acid-base theory provides a means to describe the ability of âligandsâ to coordinate to metal ions as well as the stability of metal-ligand complexes. Recall that coordination bonds are the donation/acceptance of lone pairs of electrons to empty metal orbitals to another atom; the definition of Lewis acid-base chemistry! The stability of Lewis acid-base complexes can be described through the stability constant which is analogous to the Ka that describes BrÃ¸nsted-Lowry acidities. Many metal ions can coordinate (bind) many Lewis base ligands in a step-wise fashion according to:
M + L = ML K1 = [ML]/[M][L] Eq. 1
ML + L = ML2 K2 = [ML2]/[ML][L] Eq. 2
ML2 + L = ML3 K3 = [ML3]/[ML2][L] Eq. 3
MLn-1 + L = MLn Kn = [ML4]/[ML3][L] Eq. 4
and for which the free energy for this dissociation reaction is given by:
ÎG0 = -RTLnKn Eq. 5.
where R is the gas constant and T is the temperature in Kelvin.
Figure 1: Left- EDTA. Right- Typical EDTA-transition metal complex.
The ethylene diamine tetraacetic acid ligand (EDTA, a Lewis base) provides a mechanism to probe the Lewis acid strength of various metal complexes (Figure 1). This multi-dentate ligand (multiple Lewis base sites on a single ligand) forms complexes with metals of the form:
Mn+ + Y4- = MYn-4 Eq. 6
K = [MYn-4]/[Mn+][Yn-4] Eq. 7
where Y4- is the EDTA ligand.
In this experiment, the Lewis acid strength for two different metals will be examined by measuring the stability constants for the metal-EDTA complex. Of specific interest is the how the Lewis acid strength of the metal ion correlates with:
1) atomic/ionic radii, 2) oxidation state, 3) electron configuration, and 4) polarizability.
You will determine the stability constants of both a Cu2+- and Zn2+-EDTA complexes as well as the corresponding free energy. Using these data, you will compare the relative Lewis acidities of both metals and describe differences in terms of the metal ion properties described above.
.
Chemicals
CuSO4
ZnSO4
EDTA
Murexide
Instruments
Buret
Experimental
You and your group will determine the concentration and ultimately the free energy of Cu(II) and Zn(II) using the hexadentate chelating ligand, EDTA.
You will be given a ~100 mM standardized solution of EDTA (check the label for exact concentration prior to beginning). You will need to weigh ~0.1 g per metal salt sample. Dissolve the metal salt into 50 mL of DI water. Add 2-3 drops of murexide to your metal. After properly preparing your buret with EDTA, titrate your metal with EDTA. Repeat for a total of 3 titrations per metal.
1. Approximately how much EDTA will you need for each titration assuming the metal-saltsâ masses are accurate?
2. What type of indicator is murexide and what color will its endpoint be?
Post-Lab Questions
1.
a) Using the equations given in the introduction and your titration results, what are the Kn of the EDTA-Copper complex? Show calculations.
b) Using the equations given in the introduction and your titration results, what are the Kn of the EDTA-Zinc complex? Show calculations.
2. What are the corresponding ÎG values for the EDTA-metal complexes? Show your calculations
3. Would copper be considered a hard or soft lewis acid? What about zinc? Describe what makes these metals hard or soft.
4. What trends would you predict for EDTA-metal complex Kn and ÎG values when:
a) going down a row of transition metals of the same oxidation state?
b) of the same transition metal with increasing oxidation state? (For example, Iron can exist as Fe(II), Fe(III), Fe(IV), and higher oxidation states)
5. Discussion (formal report style): Compare and contrast the concepts of Lewis and BrÃ¸nsted-Lowry acids and bases. Support these concepts with your data over the last two weeks and with research. Cite your sources.

CHEM 14A Lecture Notes - Electron Affinity, Acid Strength, Gibbs Free Energy

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